Can you put new roof decking over old decking?

When an overlay makes sense

Many older Georgia homes have 1x6 or 1x8 board decking, and if the house once wore wood shakes, those boards may have gaps between them. Asphalt shingles need a solid, continuous deck, so roofers often skin those boards with 7/16 inch OSB or 1/2 inch plywood instead of ripping every board out. The old boards add stiffness, the new panels give a flat nailing surface, and the job stays cleaner than a full strip.

When it does not

• Boards that are soft, delaminated, blackened or crumbling must come out first. Rot keeps spreading under a new layer.
• Sagging between rafters usually means the framing or deck is undersized. Adding weight on top makes it worse.
• If the existing deck is already modern plywood or OSB in good shape, a second layer adds cost without benefit.

How it should be done

Panels run across the rafters with staggered end joints, a small expansion gap at the edges, and nails long enough to pass through both layers into the framing. Nailing only into the old boards is the most common shortcut, and it lets panels creep and buckle. The overlay raises the roof surface slightly, so the drip edge, wall flashing and the shingle line at the gutters all need to be set for the new height. Attic ventilation still has to be balanced; a thicker deck does not change how much intake and exhaust the attic needs.

Your estimate should list the overlay as its own line, priced by the sheet. For material choices, see OSB vs plywood roof decking, and for where the deck fits in the system, roof anatomy explained.